The demand for greater quantities of information and data transfer to and from residential, business and other users continues to grow faster than supply can keep up with it. This information demand is being supplied in a variety of forms comprising of telephone systems of various forms, cable systems, hybrid fiber/cable systems, and wireless systems. Local Multi-point Distribution System (LMDS) and Multichannel Multipoint Distribution Service (MMDS) provide such one way and two way broadband services as broadcast video, video-on-demand, multimedia capability, interactive video, high speed remote LAN/Internet access, telephony, telecomputing, speed learning, video conferencing, electronic sales/marketing, telemedicine, home shopping and high speed computer data links as examples. Both systems can provide a wireless infrastructure to deliver broadband services from, for example, a local TELCO central office (CO), or a cable "Headend" facility. FIG. 1 illustrates such a system. The exemplary system comprises three basic components: a Headend facility, a system of base or hub stations, and a multitude or plurality of subscriber stations. The overall system is made up of a geographical structure of non-overlapping cells, wherein each geographical cell can have a large number of subscriber stations supported by one Base Station. A plurality of Base Stations are interfaced to a single Head-End.
As shown in FIG. 1, the Head End collects all signals to be distributed throughout the system thereby forming a star configuration. The Head End at the center of the star, the Base Stations surround the Head End and the subscribers surround their respective Base Stations. As Examples of signals collected, digital video may be gathered via satellite links, a telephone system interface may be provided via Class 5 switches, and/or high rate digital data networks may be interfaced via a high rate data switch. The data to/from the Head End is distributed to the system of local base stations each assigned to serve its geographical "cell" of subscriber stations.
In such point-to-multipoint systems, where a base station is receiving from multiple subscribers, it is efficacious to reduce the frequency uncertainty of the transmissions from the subscribers to the base station (heretofore called the upstream direction). For example in either burst time-division multiple access (TDMA) or frequency-division multiple access (FDMA) continuous mode , the frequency uncertainty of narrowband data transmissions may comprise multiple frequency channels, which could cause one Subscriber to interfere with another on an adjacent frequency channel. In order to avoid such inter-channel interference, a large unusable guard band is often required between adjacent frequency channels, leading to inefficient band use.
In burst mode (TDMA) broadband systems, the signals from subscribers must be acquired during each burst. In burst mode systems, each burst typically consists of a preamble for synchronization, the actual data, and a guard band. A measure of efficiency of the channel is the proportion of data to the total burst length. The larger the frequency uncertainty of the subscriber transmissions as compared to the modulation symbol rate, the longer must be the synchronization preamble at the beginning of a burst, reducing the efficiency of the channel. For a given subscriber local oscillator stability, typically around 10 parts per million (ppm) for low cost oscillators, the amount of subscriber transmission frequency error is proportional to the carrier radio frequency (RF). As a result, this problem becomes particularly acute at the higher frequency bands currently being made available for these services-the ultra high frequency (UHF) (0.3-3 GHz), super high frequency (SHF) (3-30 GHz) and extremely high frequency band (EHF) (30-300 GHz). At 10 ppm, the corresponding carrier frequency errors are 3-30 KHz, 30-300 KHz and 300-3,000 KHz for UHF, SHF, and EHF respectively. The invention is especially useful in providing a low cost solution to solve subscriber frequency error problems in the 0.3-300 GHZ range.
Typically, this problem is solved by ensuring that the frequency stability, measured in parts per million of the reference oscillators in the subscriber terminals is arbitrarily small. This results in the use of expensive oscillators which have special aging characteristics or use thermistor networks or electrically heated ovens to compensate for temperature-induced frequency drift. These solutions are orders of magnitude more expensive than desired for low cost consumer telecommunications equipment.
Another solution using inexpensive crystal oscillators is to slave the transmitter local oscillator to the receiver local oscillator via a phase-locked-loop M/N frequency synthesizer where M/N is the proportion of the transmitter frequency to the receiver frequency. This is undesirable because receiver thermal noise within the receiver carrier recovery phase looked loop causes phase noise on the transmitter local oscillator, resulting in excessive phase noise received at the head-end and hence a degradation or a lower bit-error-rate performance. In order to reduce the phase noise of the transmitter local oscillator, the noise bandwidth of the receiver phase locked loop must be made arbitrarily small, which reduces the ability of the receiver phase locked loop to track phase noise on the received signal causing a degradation in the subscriber bit error rate performance.
The object of the invention is to provide a low cost frequency control system which ensures that the frequency uncertainty of subscriber transmissions is made arbitrarily small with inexpensive crystal oscillators, without sacrificing bit error rate performance as a result of excessive phase noise.